Single use cameras are commonly sold preloaded with a cartridge of film. In order to reduce complexity and costs, a rewind mechanism is omitted from the camera. Instead, the film is prewound and during use is advanced back into the film canister. A number of approaches have been followed in loading and prewinding film cartridges for single use cameras. In some approaches, such as one taught in U.S. Pat. No. 4,972,649; film is prewound into a roll outside the camera body and then loaded. A shortcoming of these approaches is that the film roll must be handled after it is formed. This presents a risk of film damage and may add complexity to necessary equipment. In some other approaches, also taught in U.S. Pat. No. 4,972,649; the cartridge is loaded, the camera is light-tightly closed, and the film is then prewound. In still other approaches, such as taught by U.S. Pat. No. 5,311,231; the cartridge is loaded and then the rear opening of the camera body is closed and the film is prewound through a bottom opening, which is later sealed. The latter two approaches have the shortcoming that film guiding is provided primarily by the camera body, rather than loading apparatus. This places constraints on the camera body in terms of required tolerances and the like and may, in addition, slow throughput speeds. Still another approach is taught by Japanese Kokai 6-295022, European Patent Application No. 0743546-A, and Japanese Kokai 8-171180. In this approach, the film is wound onto a second spool, rather than being wound into a film roll. The back of the camera is not mandatory for guiding the film, since the second spool tends to restrain the film.
One of the risks presented in loading and prewinding film cartridges into single use camera assemblies has been damage due to pinching of the filmstrip during assembly. One source of filmstrip pinching is presented by the active light lock (sometimes referred to as "film door") of a thrust film cartridge, such as an Advanced Photo System.TM. (APS.TM.) film cartridge. The active light lock is designed to be opened and closed only when the filmstrip is fully wound into the cartridge; however, the active light lock can be closed onto an extended filmstrip. This can crimp the filmstrip causing local damage and presenting a risk of additional damage due to mishandling of the filmstrip by processing equipment and the like.
The active light lock is unlikely to close by itself; but many cameras have a subsystem that automatically closes the active light lock unless a particular condition is detected. Such subsystems commonly rely upon optical film detection and are relatively expensive. A mechanical subsystem, referred to herein as an "active light lock closer", is disclosed in U.S. Pat. Nos. 5,614,976 and 5,629,750, both to Smart et al. The active light lock closer shuts the active light lock unless at least one of the following is present: a sensor lever detects the filmstrip extending from the cartridge; or the spool is in a "safety zone", that is, the spool has a rotational orientation relative to the camera frame that is in a predetermined range in which the active light lock is held open. The rotational orientation of the cartridge spool is known since the spool and the thumbwheel are reciprocally keyed. The term "reciprocally keyed" and similar terms, used herein, refer to complementary structures on the cartridge spool and the thumbwheel that allow engagement of the thumbwheel and spool in only a single rotation orientation relative to each other. For example, Advanced Photo System.TM. cartridge has a spool having a generally cylindrical socket with a notch or keyway extending axially part way along the socket. A matching thumbwheel has generally cylindrical shaft with an axially extending key complementary to the keyway of the spool. A limitation of the active light lock closer is that, in the completed camera, the back of the camera presses the filmstrip against the sensor lever. Without the back, during camera assembly; the sensor lever is, at best, undependable and the retaining the active light lock open is solely a function of spool orientation.
It would thus be desirable to provide apparatus and methods in which the active light lock of a film cartridge in a camera frame assembly, is dependably kept open, when film is present, in a camera frame assembly lacking a camera back.